US20040168771A1 - Plasma reactor coil magnet - Google Patents

Abstract

A method for processing a workpiece is carried out with a plasma derived from a process gas in a plasma chamber of a plasma processing apparatus during a plasma processing operation. The apparatus includes an array of electromagnets mounted circumferentially around the plasma chamber. The method comprises generating a plasma from a process gas within the chamber and causing plasma particles to strike the workpiece, selecting distributions of current signals for the electromagnets, and applying each selected distribution to the electromagnets to impose more than one magnetic field topology on the plasma during the plasma processing operation.

Description

• This is a continuation of International Application No. PCT/US02/27978, filed on Sep. 4, 2002, which, in turn, claims the benefit of U.S. Provisional Application No. 60/318,890, filed Sep. 14, 2001, the contents of both of which are incorporated herein in their entirety by reference.[0001]
FIELD OF THE INVENTION
• The present invention relates to plasma processing systems and more particularly to a method and apparatus for using a magnetic field imposed on a plasma to control plasma characteristics to improve plasma processing of a workpiece.[0002]
BACKGROUND OF THE INVENTION
• A plasma is a collection of charged particles that may be used to remove material from or deposit material on a workpiece. Plasma may be used, for example, to etch (i.e., remove) material from or to sputter (i.e., deposit) material on a semiconductor substrate during integrated circuit (IC) fabrication. A plasma may be formed by applying a radio frequency (RF) power signal to a process gas contained in a plasma chamber to ionize the gas particles. The RF source may be coupled to the plasma through a capacitance, through an inductance, or through both a capacitance and an inductance. Magnetic fields may be imposed on the plasma during plasma processing of a workpiece to improve plasma characteristics and thereby increase control over the plasma processing of the workpiece.[0003]
• Magnetic fields are sometimes used during the plasma processing of a workpiece to contain the plasma within the chamber or to change plasma properties during plasma processing. Magnetic fields may be used, for example, to contain the plasma within the chamber, thereby reducing plasma loss to the chamber walls, and to increase plasma density. Increasing plasma density increases the number of plasma particles striking the workpiece, which improves the processing of the workpiece by, for example, decreasing the processing time required to etch a workpiece. Containment of the plasma using magnetic fields also prevents plasma particle deposition on surfaces within the chamber such as chamber wall surfaces and electrode surfaces.[0004]
• Magnetic fields are also used to increase the uniformity of the distribution of plasma within the chamber. Non-uniform distribution of plasma within a plasma chamber is undesirable because non-uniform distribution may result in non-uniform processing of the workpiece. Non-uniformly distributed plasmas may, in some situations, result in plasma-induced damage to the workpiece being processed in the chamber.[0005]
• Arrays of either permanent magnets or electromagnets are sometimes used to impose a magnetic field on the plasma. An array of permanent magnets can be arranged, for example, so that they impose a magnetic field on the plasma within the interior of the chamber, or, alternatively, they can be arranged and moved (by rotation with respect to the chamber, for example) so that they impose a rotating magnetic field on the plasma, which improves plasma uniformity.[0006]
SUMMARY OF THE INVENTION
• The present invention includes methods and apparatuses for utilizing magnetic fields to control the processing of a workpiece with the plasma.[0007]
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a schematic diagram of an example plasma processing system for illustrating the present invention, the plasma processing system showing a workpiece and plasma within a plasma chamber of a plasma processing apparatus and showing an outer flux conducting structure and an array of electromagnets surrounding the processing chamber;[0008]
• FIG. 2 is a schematic top plan view of a portion of the apparatus of FIG. 1, FIG. 2 showing the processing chamber, a lower electrode, the outer flux conducting structure and the array of electromagnets surrounding the processing chamber and showing a magnetic cross field topology imposed on the interior of the chamber;[0009]
• FIG. 3 is identical to FIG. 2 except showing a magnetic bucket field topology imposed on the interior of the chamber;[0010]
• FIG. 4 is a schematic representation of an example power supply circuit for supplying an array of magnets with electrical power;[0011]
• FIG. 5 is a schematic representation of a second example power supply circuit for supplying an array of magnets with electrical power;[0012]
• FIG. 6 is a schematic view similar to FIG. 3 except showing a bucket field topology imposed on the processing chamber by two systems of electromagnets; and[0013]
• FIG. 7 is a graph showing current flows in four adjacent electromagnets of the apparatus of FIG. 6.[0014]
DETAILED DESCRIPTION OF THE INVENTION
• FIG. 1 shows a schematic representation of an example of a plasma processing apparatus (or reactor)[0015]10 of aplasma processing system12. Theplasma processing apparatus10 includes aplasma chamber14, which provides aninterior region16 for containing and supporting a plasma. A plurality of electrodes may be mounted within thechamber14 in plasma generating relation to one another and to a process gas within thechamber14. The electrodes are energized to generate a plasma from the process gas within thechamber14. To facilitate the description of the invention, only two electrode assemblies are included in theapparatus10. Specifically, afirst electrode assembly18 is mounted on a first side of the chamber14 (in an upper portion of theinterior16 of thechamber14 in the example apparatus10). A second electrode assembly in the form of achuck electrode assembly20 is mounted on a second side of thechamber14 opposite the first side of the chamber14 (in a lower portion of thechamber interior16 in the example apparatus10) in a position spaced from thefirst electrode assembly18.
• The[0016]first electrode assembly18 may include a plurality of electrode segments, each segment being electrically isolated from the other segments and each segment being independently powered by an associated RF power source and independently supplied with a selected process gas for transmission at a predetermined rate into the interior of the plasma chamber. To facilitate the description of the present invention, however, thefirst electrode assembly18 is in the form of a single showerhead-type electrode. Thefirst electrode assembly18 includes an inner chamber22 (indicated schematically by a broken line in FIG. 1) that is in pneumatic or fluidic communication with agas supply system24 through a gas supply line. A selected gas (or gasses) may be supplied to theelectrode assembly18 to purge thechamber14, for example, or to serve as a process gas (or source gas) for plasma formation in thechamber interior16. The process gas is transmitted from thechamber22 into theinterior16 of theplasma chamber14 through a plurality of gas ports (not shown). The flow of gas through the ports of the first electrode is indicated by a series of directional arrows G.
• The first and[0017]second electrodes18,20 are electrically communicated through associatedmatching networks30,32 to respectiveRF power sources34,36 which provide voltage signals V_B1, V_B2, respectively, to the associatedelectrodes18,20.Matching networks30,32 may be inserted between respectiveRF power sources34,36 in order to maximize the power transferred to the plasma by therespective electrode assemblies18,20. Alternately, thematching networks30,32 may be coupled tocontrol system60.
• Each[0018]electrode assembly18,20 may be independently cooled by a fluid that circulates from acooling system38 through afluid chamber39,41 (indicated by a broken line) in eachelectrode assembly18,20, respectively, and then back to the cooling system. Theplasma processing apparatus10 further includes avacuum system40 in pneumatic or fluidic communication with theplasma chamber16 through a vacuum line. Theplasma processing apparatus10 optionally includes avoltage probe44,46 in the form of a pair of electrodes capacitively coupled to the transmission lines between the associatedRF power sources34,36, respectively, and the associatedelectrode assembly18,20, respectively. (An example voltage probe is described in detail in commonly assigned pending U.S.application 60/259,862 (filed on Jan. 8, 2001), and it is incorporated in its entirety herein by reference.) Theplasma processing apparatus10 optionally includes anoptical probe48 for determining plasma characteristics and conditions based on spectral and optical properties of the plasma.
• A system or array of[0019]electromagnets51 are mounted circumferentially around theplasma chamber14. Theelectromagnets51 are operable to impose one or more magnetic fields on a plasma during a plasma processing operation on a workpiece. The imposition of a magnetic field improves the condition of the plasma and thereby improves the processing of the workpiece.
• FIG. 2 shows an example of an arrangement of the plurality of[0020]electromagnets51 with respect to theplasma chamber14. Theexample apparatus12 includes twelve electromagnets, designated51A-L. Eachelectromagnet51 shown is in the form of a coil magnet that includes a coil of an electrically conductive material. Each coil is in electrical communication with an electrical power source53 (shown schematically in FIG. 1).
• Each[0021]coil magnet51 of a particular array may be provided by a coil of conductive material wound on an air core (not shown) or, alternatively, may be provided by a coil of conductive material wound around a core55 (partially visible in FIG. 1) of, for example, a magnetically permeable material. Eachcore55 may have a cylindrical cross section (as shown) or, alternatively, may have an arbitrary elongated cross section (with the longer dimension extending in the vertical direction in the example apparatus10). The axis of eachcoil magnet51 is radially aligned with theplasma chamber14. That is, the axis of eachcoil magnet51 extends radially from an imaginary axis that extends (vertically in the example reactor10) between theelectrode assemblies18,20 through the center of theplasma chamber14. An outerflux conducting structure57 may be mounted in surrounding relation to the array ofcoil magnets51 as best seen in FIG. 2. Eachcoil magnet51 and eachcore55 is in magnetic flux communication with theflux conducting structure57. An example of theflux conducting structure57 is an annular wall structure. Both theouter wall structure57 and thecore55 of eachcoil magnet51 may be constructed of a magnetically permeable material such as iron. Each core55 may be integrally formed on theouter ring structure57 or may be formed separately from theouter wall structure57 and then mounted on theouter ring structure57.
• It can be appreciated from FIG. 2 that each[0022]coil magnet51 and its associatedcore55 extends in a radial direction between theouter ring structure57 and thewall structure59 of theplasma chamber14. In theexample apparatus10, thewall structure59 is cylindrical and comprises the side wall of theprocessing chamber14. Thewall structure59 of theplasma chamber14 may be constructed of either a suitable dielectric material or a suitable metallic material. If thewall structure59 is constructed of a metallic material, a non-magnetic metallic material is used in the construction so that thewall structure59 does not interfere with a magnetic field imposed on a plasma within theplasma chamber14 by thecoil magnets51.
• The array of magnets in the[0023]example apparatus10 is vertically aligned with the plasma in FIG. 1, but this vertical positioning is an example only. The array of magnets could have any vertical position with respect to the processing chamber and the structures (the electrodes, for example) and materials (the workpiece or plasma, for example) contained therein. For example, theapparatus10 could be constructed and arranged so that the array of magnets are vertically aligned with the top of the workpiece, aligned with the center of the workpiece, slightly above the workpiece, for example, or aligned with the vertical center of the plasma, or slightly above or below the plasma, for example.
• A[0024]control system60 of theplasma processing apparatus10 is electrically communicated to various components of theapparatus10 to monitor and/or control the same. Thecontrol system60 is in electrical communication with and may be programmed to control the operation of thegas supply system24,vacuum system40, thecooling system38, thevoltage probe44,46, theoptical probe48, eachRF power source34,36 and thepower source53. Thecontrol system60 may send control signals to and receive input signals (feedback signals, for example) from theprobes44,46,48 andsystem components24,34,36,38,40,53. Thecontrol system60 may monitor and control the plasma processing of a workpiece. By controlling thepower source53, thecontrol system60 is able to control the transfer of electrical power to each coil magnet comprising the array ofcoil magnets51, and thereby control the properties of the magnetic field imposed on the plasma.
• The[0025]control system60 may be provided by a computer system that includes a processor, computer memory accessible by the processor (where the memory is suitable for storing instructions and data and may include, for example, primary memory such as random access memory and secondary memory such as a disk drive) and data input and output capability communicated to the processor.
• The methods of the present invention are illustrated with reference to the example[0026]plasma processing system12. The operation of theplasma processing system12 can be understood with reference to FIG. 1. A workpiece (or substrate)62 to be processed is placed on a support surface provided by thechuck assembly20. Thecontrol system60 activates thevacuum system40 which initially lowers the pressure in theinterior16 of theplasma chamber14 to a base pressure (typically 10⁻⁷to 10⁻⁴Torr) to assure vacuum integrity and cleanliness for thechamber14. Thecontrol system60 then raises the chamber pressure to a level suitable for forming a plasma and for processing theworkpiece62 with the plasma (a suitable interior pressure may be, for example, in the range of from about 1 mTorr to about 1000 mTorr). In order to establish a suitable pressure in thechamber interior16, thecontrol system60 activates thegas supply system24 to supply a process gas through the gas inlet line to thechamber interior16 at a prescribed process flow rate and thevacuum system40 is throttled, if necessary, using a gate valve (not shown). The process gas may flow through ports in the first electrode assembly as indicated in FIG. 1 by arrows G.
• The particular gas or gases included in the[0027]gas supply system24 depends on the particular plasma processing application. For plasma etching applications, for example, thegas supply system24 may supply chlorine, hydrogen-bromide, octafluorocyclobutane, or various other gaseous fluorocarbon compounds; for chemical vapor deposition applications, thesystem24 may supply silane, ammonia, tungsten-tetrachloride, titanium-tetrachloride, or like gases. A plasma may also be used in chemical vapor deposition (CVD) to form thin films of metals, semiconductors or insulators (that is, conducting, semiconducting or insulating materials) on a semiconductor wafer. Plasma-enhanced CVD uses the plasma to supply the required reaction energy for deposition of the desired materials.
• The[0028]control system60 then activates theRF power sources34,36 associated with the first andsecond electrode assemblies18,20. TheRF power sources34,36 may provide voltages to the associatedelectrodes18,20 at selected frequencies. Thecontrol system60 may, during a plasma processing operation, independently control theRF power sources34,36 to adjust, for example, the frequency and/or amplitude of the voltage at which eachsource34,36 drives the associatedelectrode assembly18,20.
• The[0029]RF power sources34,36 may be operated to convert the low-pressure process gas to a plasma. Thepower sources34,36 may be operated, for example, to cause an alternating electric field to be generated between the first andsecond electrodes18,20 which induces an electron flow between theelectrodes18,20. Electrons, for example, are accelerated in this electric field and the flow of heated electrons in the field ionizes individual atoms and molecules of the process gas by transferring kinetic energy thereto through multiple collisions between the electrons and the gas atoms and molecules. This process creates aplasma54 that is confined and supported within thechamber14.
• Because each[0030]RF power source34,36 is independently controllable by thecontrol system60, either power source may be operated to have a relatively low frequency (i.e., a frequency below 550 KHz), an intermediate frequency (i.e., a frequency around 13.56 MHz), or a relatively high frequency, around 60 to 150 MHz. In an example of an etch reactor, theRF power source34 for thefirst electrode assembly18 can be driven at a frequency of 60 MHz and theRF power source36 for thesecond electrode assembly20 can be driven at a frequency of 2 MHz. In order to improve the performance of the aforementioned reactor, or, more generally, a plasma processing device having one or more electrodes that are driven at one or more frequencies, thecontrol system60 can be programmed and operated to impose one or more magnetic fields on the plasma during processing of the workpiece to control the characteristics (such as, for example, magnetic field topology and orientation, magnetic field strength, magnetic field duration and so on) of the magnetic fields.
• The invention allows a large number of possible magnetic field topologies to be generated using a single array of[0031]magnets51 having no moving parts. FIGS. 2 and 3 show two magnetic field topologies that can be imposed on the plasma54 (theplasma54 being shown schematically in FIG. 1 only) using the magnet system. FIG. 2 shows a cross field topology and FIG. 3 shows a magnetic bucket field topology.
• The cross field topology illustrated has nonlinear (i.e., arcuate) magnetic fields lines. The cross field topology may be used to improve the uniformity of the plasma. Increasing the plasma uniformity increases the process uniformity both for a[0032]single substrate62 and also increases process uniformity among a plurality of substrates processed in succession by theapparatus10. The array ofelectromagnets51 may be operated to rotate the cross field topology in a manner described below. A magnetic bucket topology (FIG. 3) may be imposed on the plasma to reduce plasma wall loss and to increase plasma density.
• An example of a[0033]circuit68 for realizing theelectrical power source53 for powering thecoil magnets51A-L to create a desired magnetic field topology is shown schematically in FIG. 4. Specifically, each of a series ofarbitrary waveform generators70A-L may be electrically communicated to arespective coil magnet51A-L (not shown in FIG. 4) of the system of electromagnets through an associatedamplifier71A-L.
• Each[0034]arbitrary waveform generator70A-L may be electrically communicated to the control system60 (through electrical connections that are not shown in FIG. 4). Thecontrol system60 can be programmed to control each of thearbitrary waveform generators70A-L independently of one another to generate from each a current waveform of arbitrary shape, magnitude and phase for transmission to the associatedcoil magnet51A-L to polarize the same and to create the magnetic field that is imposed on the plasma. All of the arbitrary waveform generators70 may be phase locked to a single low powerreference signal source72. Each generator70 is capable of shifting the phase of its output relative to the reference signal from72.
• The power source arrangement of FIG. 4 enables the control system[0035]60 (acting through the series ofarbitrary waveform generators70A-L) to supply eachcoil magnet51 with a current waveform having a wave shape, amplitude, phase and period that is independent of the current waveforms generated by all other arbitrary waveform generators in the series. Thus, the reference signal from thereference signal source72 is used to synchronize the current waveforms transmitted from the system of arbitrary waveform generators to thecoil magnets51. Thecontrol system60 can independently program each arbitrary waveform generator70 to generate a different waveform with the starting phase locked to the reference signal fromsource72. This arrangement provides great flexibility in imposing, for example, two or more magnetic field topologies on the plasma. This arrangement allows thecontrol system60 to impose, for example, two magnetic field topologies in succession with one another during a plasma processing operation on a particular substrate. These two topologies can be the same as one another or can be different from one another. A topology may be stationary or may be rotated.
• For example, this arrangement (that is, using a separate arbitrary waveform generator for each coil magnet) allows an operator to program the[0036]control system60 to impose on the plasma a stationary magnetic field topology (azimuthally, for example) and a rotating magnetic field topology, during a processing operation. Each imposed field topology can be selected to achieve a particular change in the plasma. For example, the rotating cross field topology may be applied to improve plasma uniformity. As another example, this arrangement also allows the waveforms to be generated such that even though the imposed magnetic field is rotating, there is a localized, field (e.g., a low- or a high-field region) imposed at a particular location within the processing chamber. This localized field may be used to correct for azimuthal variations of plasma properties which result from, for example, non-axisymmetric gas injection and the pumping of the plasma, and so on.
• Another circuit[0037]76 that can be used as apower source53 is shown schematically in FIG. 5. A singlearbitrary waveform generator77 drives a series ofamplifiers71A-L that each supply current to an associated coil magnet (not shown in FIG. 5). A phase delay circuit78 is coupled between the arbitrary waveform generator and all but one of the amplifiers. Essentially the same signal is sent to eachcoil magnet51A-L, the only difference being that the signals may be out of phase with one another because of the presence of the phase delay circuits. Therefore, the circuit76 can be used in instances in which the current waveforms to be applied to thecoil magnets51 have identical wave shapes and periods but differ from one another in phase. The power supply circuit76 can provide a rotating magnetic field topology or a magnetic field topology that changes angular orientation. The topology of the field that is produced and rotated by the power source circuit76 depends on several factors including the shape of the current waveforms transmitted to the coil magnets, the number of and the relative positions of the coil magnets in the coil magnet system, relative field strength of eachcoil magnet51 and the phase difference between the current waveform signals. Thecontrol system60 can be programmed to control the arbitrary waveform generator70 of circuit76 to produce, for example, a rotating cross field topology having nonlinear (e.g., arcuate) fields lines.
• Operation[0038]
• The[0039]control system60 can produce steady currents in any (or all)coil magnets51 or can produce a time changing current in any (or all)coil magnets51A-L (using, for example, the power supply circuitry68). The distribution of steady and/or time-varying currents passing through thecoil magnets51 determines the topology of the magnetic field imposed on the plasma and determines the change in time of the magnetic field topology. Appropriate current waveforms can be sent to thecoil magnets51 to cause the magnetic field imposed on the plasma to rotate, for example.
• The current waveform applied to each[0040]coil magnet51A-L radially polarizes each coil. During radial polarization, opposite ends of each coil magnet assume respective North and South magnetic polarities. Generally, the magnetic fields lines extend between opposite poles of thecoil magnets51. The direction of current flow in each coil determines the polarity of each coil magnet. The magnitude of the current flowing through the coil magnet determines the strength of the magnetic field produced by each coil magnet, and therefore the strength of the magnetic field imposed on the plasma.
• Other arrangements of the array of magnets are possible. For example, although the axis of each[0041]coil magnet51 extends radially from an imaginary axis that extends between theelectrode assemblies18,20 in theexample reactor10, other arrangements are possible. For example, eachcoil magnet51 could be oriented so that its axis is “tangential” to thereactor10. Each tangentially oriented coil could be an air coil or could be wound around a core material. When each coil is wound around a core of material, each core could be a separate structure or form part of a continuous structure such as a ring or yoke.
• This tangential arrangement has several disadvantages (relative to the radial alignment in the example reactor[0042]10). For example, when a radially extending array of electromagnets are used to generate magnetic fields, most of the magnetic flux lines enter the chamber. When a tangential arrangement is used, however, most of the magnetic flux field lines tend to flow around the exterior of theplasma chamber14, particularly when the coils are wound around a yoke surrounding the chamber, and a relatively small amount “leaks” or “fringes” out of the side of each tangentially arranged coil and enters theplasma chamber14. Thus, the tangential arrangement relies on the fringing fields on one side of each tangential coil to impose a magnetic field on the plasma in the chamber. Because a magnet system that utilizes a tangential arrangement of the coils relies on fringing fields to impose a magnetic field on a plasma, more power is required to create a particular topology having a particular field strength relative to the use of a radial arrangement to create the same field topology having the same strength. A radially arranged magnet system utilizes less current than required by a comparable tangentially arranged magnet system. Because a tangential arrangement relies on field lines emerging from the sides of each coil, each coil emits field lines toward the chamber and field lines out the opposite side, for example, away from the chamber. An outer surrounding structure is also needed to shield the area surrounding the plasma processing apparatus from the magnetic field. When the tangentially oriented electromagnets are wound around a yoke, for example, a second permeable shield is needed if the area surrounding the apparatus is to be shielded from magnetic fields. A second permeable shield or other flux shielding structure is not required in the example arrangement shown in FIGS. 1 and 2, for example, because thestructure57 performs both a flux transmitting function and a shielding function.
• FIGS. 2, 3 and[0043]6 illustrate examples of magnetic field topologies that can be imposed on the plasma using thecoil magnets51A-L. The direction of the current flowing through eachcoil magnet51A-L is indicated by a directional arrow in FIGS. 2, 3 and6. The relative magnitude of the current in each coil magnet is roughly indicated by the relative size of the directional arrows in FIGS. 2, 3 and6. Absence of a directional arrow indicates an instantaneous current of zero magnitude in the associatedcoil magnet51. Theiron ring structure57 closes the magnetic field lines for each topology.
• A rotating cross field topology can be imposed on the plasma utilizing, for example, either[0044]power supply circuit68 or76. For example, a complex current waveform can be fed to eachcoil magnet51 that is phase shifted with respect to the previous coil in a rotational direction that is opposite the direction of magnetic field topology rotation. This method allows the cross field topology to be rotated without mechanically moving any of the coil magnets.
• FIG. 2 shows a rotating cross field topology at a particular instant in time. At this instant, the[0045]coil magnets51A and51B have currents that are oppositely directed to one another and are of relatively high magnitude, thecoil magnets51L and51C have currents that are oppositely directed to one another and are of lesser magnitude than the currents incoil magnets51A and51B, and the coil magnet pairs51K and51D,51J and51E, and51I and51F have oppositely directed currents of successively lesser magnitude (as indicated by the relative size of the directional arrows). Nonlinear magnetic fields lines extend generally between the coils of each pair of coil magnets as indicated by the arcuate arrows in theprocessing chamber14. Thecoil magnets51H and51G may have instantaneous currents of zero magnitude (depending, for example, on the exact field that one is trying to impose).
• It can also be appreciated from FIG. 2 that magnetic field lines extend generally from[0046]coil magnets51A,51L,51K,51J and51I on one side of the chamber to respective associatedcoil magnets51B,51C,51D,51E and51F on an opposite side of the chamber. The decreasing magnitude of the currents (on opposite sides of the chamber) creates, in effect, a magnetic field gradient of increasing strength from the approximately eleven o'clock azimuthal position to approximately the five o'clock azimuthal position. This gradient can help compensate for ExB drift. ExB drift can occur if a homogeneous field crosses aplasma chamber14 parallel to the workpiece while an electric field perpendicular to the workpiece is present in the chamber. The vector product of these electromagnetic fields is parallel to the workpiece and perpendicular to both sets of field lines. This results in having the electrons directed in the direction of the vector product (i.e., the “preferred” direction) which causes the plasma to be denser in one area (or “corner”) of the plasma chamber. This results in a nonuniformity of the processing of the workpiece, which is undesirable. To correct for this ExB drift, the magnetic field topology is rotated. If the magnetic field topology is uniform, however, rotating the field merely causes the “hot spot” (area of relatively high electron density) to rotate around the periphery of the plasma. To correct for this effect, the field lines of the magnetic field topology are curved which causes the electrons to “fan out” sufficiently to reduce the hot spot effect.
• FIG. 3 shows a bucket-type field topology (or bucket field topology) which forms a magnetic “bucket” around the walls of the[0047]chamber14. This topology produces arcuate lobes of magnetic field lines that extend toward the center of the chamber. These lobes tend to concentrate the plasma in the center of the chamber. This has a number of benefits including, for example, tending to reduce the number of plasma particles striking the chamber side wall and other surfaces within thechamber14 and increasing plasma density (by confining it to a smaller volume of space). The greater the plasma density, the faster the rate of etching or deposition, for example. Faster processing of the workpiece increases commercial productivity during, for example, semiconductor fabrication.
• As shown in FIG. 3, the bucket field topology can be achieved by conducting equal currents of opposite polarity (that is, currents of opposite direction) to pairs of[0048]adjacent coil magnets51 of the array. Thereactor12 may also be constructed to provide magnetic field lines having a bucket topology that rotates or oscillates.
• A schematic view of an[0049]apparatus80 for imposing a rotating bucket field topology on the plasma is shown in FIG. 6. Theapparatus80 is identical toapparatus12 except for the number of coil magnets mounted around thechamber14 thereof. Identical structures between the twoembodiments12 and80 are identified with identical reference numbers and are not commented upon further. The number of coil magnets mounted around thechamber14 determines the resolution of the magnetic field produced by the magnet system. That is, the more coil magnets that are positioned circumferentially around a chamber, the more “finely” the bucket field topology covers the interior of the chamber wall. To better control the “peripheral” magnetic field (that is, the portion of the magnetic field that is adjacent the wall), a relatively large number of relatively smaller coils are mounted around thechamber14 ofapparatus80. When a fine resolution field is required, the inner ends ofadjacent coil magnet51 cores almost touch one another as shown in FIG. 6. Because theapparatus80 has twice the number ofcoil magnets51 mounted around its chamber compared toapparatus12, for example, theapparatus80 can be operated to achieve a bucket field topology that is finer resolution than the bucket field topology achieved usingapparatus12. The number of coils utilized depends on the resolution of the field that is required. Generally, the greater the number of magnets, the finer the field resolution.
• The lobe length can be increased by operating the magnets in pairs, in threes, and so on. That is, when the[0050]electromagnets51 are operated in “pairs” to produce a bucket field topology, at each instant the magnitude and direction of the current incoils51A and B are identical to one another. Similarly, the magnitude and direction of the current in coils5 IC and D are identical to one another. Thus, coils51A and B (and coils51C and D and so on) function, in effect, as a single coil. The longer the lobes of the bucket field topology extend into the chamber, the more the plasma is “squeezed” into the center of theplasma chamber14, thereby raising plasma density and reaction rate.
• The[0051]apparatus80 can also be operated (using the circuit76 of FIG. 5, for example) to produce a “rotating” or oscillating bucket field topology that has the same resolution as the non-rotating bucket field topology illustrated in FIG. 3 but which produces a series of overlapping lobe patterns which tend to more uniformly “squeeze” the plasma (relative to the magnetic field topology of FIG. 3). The bucket field topology produced according to the example method described below is also advantageous because at all times at least some location has a non-zero instantaneous field strength. That is, the imposed field is always non-zero at some location in the processing chamber at each point in time. Oscillating or rotating the bucket field is advantageous because it prevents the magnetic field lines from always striking the wall (or walls) of the processing chamber at the same place (or places). If the bucket magnetic field lines are not rotated and therefore strike a wall, for example, at the same locations, this can cause plasma particles to be directed along the field lines into the wall that these locations which can result in degradation of the wall material at these locations. This local degradation of wall material from a stationary bucket magnetic field can happen, for example, in places between the lobes where the field lines from adjacent lobes enter thechamber wall14 together. Thus, it can be understood that while the imposed bucket magnetic field can be made to be static, can be made to oscillate or can be made to rotate, it may not be desirable to impose a static bucket (or other type) magnetic field on the plasma for a prolonged period because this may result in localized damage to the walls of the processing chamber. FIG. 6 shows an instantaneous view of the electrical currents and magnetic field in theapparatus80 when theapparatus80 is operated to produce a rotating bucket field topology. FIG. 7 shows a graphic representation of the magnitudes of the currents flowing through fourcoil magnets51 over time while the example rotating bucket field topology is being produced. The rotating bucket field topology inapparatus80 has essentially the same field resolution as is imposed utilizing theapparatus12.
• The[0052]coil magnets51A-X are essentially operated as two separate magnet systems that each provide a bucket field topology independently of the other magnet system. The first magnet system includes51A,51C,51E,51G,51I,51K,51M,510,51Q,51S,51U, and51W, and the second magnet system includes the remainingcoil magnets51. The graph of FIG. 7 shows the currents throughcoil magnets51A-D. It can be appreciated that the current waveforms in adjacent coil magnets (51A and51B, for example) are ninety degrees out of phase with one another. The currents in every other coil magnet (such ascoil magnets51A and C, for example) are one hundred and eighty degrees out of phase with one another.
• FIG. 6 shows the magnetic field lines that occur at a time=t[0053]_x. The time t_xis also indicated on the graph of FIG. 7. At time t_xone set of a coil magnets (the set that includes51B and51D) each have maximum current and the other set of coil magnets (the set that includes51A and51B) each have a current of zero magnitude. Adjacent coils in each set (51B and51D, for example) have oppositely directed currents as indicated by the oppositely directed current directional arrows in FIG. 6 and by the graph of FIG. 7. It can be appreciated from FIG. 7 that each current waveform is sinusoidal. It can also be appreciated from the graph of FIG. 7 that the magnetic field produced by the rotating (or oscillating) bucket field topology does not vanish at any point during the plasma processing operation because the currents are never zero in allcoil magnets51A-X, at any instant.
• The structure and operation of the[0054]apparatus80 is an example only. It is contemplated to construct an apparatus that includes three or more independent magnetic systems to produce, for example, three or more rotating magnetic field topologies.
• One or more magnetic field topologies can be imposed on the plasma during the processing of a particular workpiece (such as a semiconductor, as an example) processing quality and yield. For example, selected magnetic field topologies can be imposed on the plasma during an etching operation (or, alternatively, a deposition operation) in which a pattern is etched on a surface of a wafer of semiconductor material. Because a system of arbitrary waveform generators and[0055]coil magnets51 may be used to create the magnetic fields, and because the arbitrary waveform generators can be controlled by thecontrol system60, a manufacturer is able to select an appropriate magnetic field topology (or magnetic field topologies) for a particular semiconductor material and a particular semiconductor etching (or deposition) application. The determination of the optimal combination of magnetic field topologies for a particular application may be done experimentally. That is, particular current waveforms can be fed to selected coil magnets of one or more magnet systems during processing of a particular type of wafer and the results examined. The quality of the results of the etching/deposition can be correlated with or examined in light of the magnetic field topologies used in the etching/deposition process. If damage to the workpiece occurs, for example, or if the processing results are not uniform, the distributions of current waveforms fed to thecoil magnets51 can be changed (by reprogramming the control system60) to, for example, change the topology (or topologies), strength, gradient, period, and so on of the magnetic field topologies imposed on the plasma.
• When a semiconductor is processed in a plasma chamber, the semiconductor is susceptible to damage caused by nonuniform concentrations (either areas of high concentration or low concentration) of electrons in the plasma. Most of the damage that occurs due to nonuniformities in the concentration of the plasma occurs towards the end of a processing operation. Two or more magnetic fields topologies can be used during the processing of a workpiece (such as a semiconductor) to mitigate against the damage that may occur from plasma nonuniformities. During the first portion of a processing operation, when the workpiece is relatively unsusceptible to damage from plasma density nonuniformities, one or more bucket field topologies may be imposed on the plasma to increase plasma density and thereby increase the rate of processing. By increasing plasma density in the early part of a processing operation, therefore, material can be etched away from the workpiece faster, for example, and then, toward the end of the process, when it becomes risky to run at such a high processing rate, another magnetic field or fields can be imposed on the processing chamber to improve plasma uniformity during the final critical stages of the processing operation. As another example, a bucket field topology having relatively large lobes can be imposed on the plasma during the initial stages of processing, then a bucket field topology having intermediate sized lobes can be imposed on the plasma, and then a bucket field topology having relatively small lobes can be imposed on the plasma. By decreasing the size of the lobes (either in steps or continuously over time) of the bucket field topology during a processing operation, the density of the plasma can be gradually reduced as processing occurs. During the final critical stages of plasma processing, a rotating cross field topology having curved field lines can be imposed on the plasma to increase plasma uniformity during the final critical stages of processing.[0056]
• Localized nonuniformities can occur in a plasma for a number of known reasons including, for example, because of nonuniform gas injection, nonuniform RF excitation fields being applied to the plasma, nonuniform pumping within the plasma chamber, and so on. Because each coil magnet can be driven by an independent arbitrary waveform generator, the controller can be programmed to control the distribution of currents sent to the array of magnets to compensate for a local nonuniformity in the plasma. Thus, the controller can be programmed to create a rotating field that provides a localized nonuniformity in the imposed magnetic field to compensate for the density nonuniformity occurring in the plasma.[0057]
• It will be understood that while the electrodes of a plasma chamber were described as each being driven by an associated voltage source, this does not imply that each electrode has to be driven by the associated voltage source. Thus, for example, it is possible for one or the other of the pair of[0058]electrodes18,20 of thesystem10 to be constantly at ground level or at any other static (i.e., unchanging) voltage level during processing.
• The many features and advantages of the present invention are apparent from the detailed specification and thus, it is intended by the appended claims to cover all such features and advantages of the described method which follow in the true spirit and scope of the invention. Further, since numerous modifications and changes will readily occur to those of ordinary skill in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described. Moreover, the method and apparatus of the present invention, like related apparatus and methods used in the semiconductor arts that are complex in nature, are often best practiced by empirically determining the appropriate values of the operating parameters, or by conducting computer simulations to arrive at best design for a given application. Accordingly, all suitable modifications and equivalents should be considered as falling within the spirit and scope of the invention.[0059]

Claims (26)

What is claimed is:

1. A method for processing a workpiece with a plasma derived from a process gas in a plasma chamber of a plasma processing apparatus during a plasma processing operation, the apparatus including an array of electromagnets mounted circumferentially around the plasma chamber, the method comprising:

generating a plasma from a process gas within the chamber and causing plasma particles to strike the workpiece;

selecting distributions of current signals for said electromagnets; and

applying each said selected distribution to said electromagnets to impose more than one magnetic field topology on the plasma during the plasma processing operation.

2. The method ofclaim 1, wherein at least one magnetic field topology is a non-rotating magnetic field topology.

3. The method ofclaim 1, wherein at least one magnetic field topology is a rotating magnetic field topology.

4. The method ofclaim 3, wherein the at least one rotating magnetic field topology corrects a non-uniformity in plasma density of said plasma while said at least one rotating magnetic field topology is imposed on said plasma.

5. The method ofclaim 4, wherein the at least one rotating magnetic field topology is a cross field topology.

6. The method ofclaim 5, wherein the magnetic field lines of the cross field topology are non-linear.

7. The method ofclaim 2, wherein the at least one non-rotating magnetic field topology is a bucket field topology.

8. The method ofclaim 1, wherein the more than one magnetic field topologies include a cross field topology and a bucket field topology.

9. The method ofclaim 1, wherein the applying includes supplying the current signals such that a bucket field topology is imposed on the plasma during a first portion of plasma processing operation and a cross field topology is imposed on the plasma during a second portion of plasma processing operation.

10. The method ofclaim 9, wherein a plurality of bucket field topologies are imposed on the plasma to decrease plasma density at a predetermined rate during the first portion of the plasma processing operation.

11. The method ofclaim 10, wherein the magnetic field lines of the cross field topology are nonlinear.

12. The method ofclaim 11, wherein the cross field topology corrects a nonuniformity of said plasma.

13. The method ofclaim 1, wherein at least one magnetic field topology changes angular orientation during processing.

14. The method ofclaim 13, wherein said at least one magnetic field topology that changes angular orientation during processing changes angular orientation by rotating.

15. A method for processing a workpiece with a plasma derived from a process gas in a plasma chamber of a plasma processing apparatus during a plasma processing operation, the apparatus including an array of electromagnets mounted circumferentially around the plasma chamber, the method comprising:

generating a plasma from a process gas within the chamber and causing plasma particles to strike the workpiece; and

supplying a distribution of current signals to said electromagnets so that said electromagnets impose a rotating bucket magnetic field topology on the plasma during the plasma processing operation.

16. The method ofclaim 15, wherein the array of electromagnets comprises a first system of electromagnets and a second system of electromagnets, each electromagnet of each system being positioned between a pair of electromagnets of the other system.

17. The method ofclaim 16, wherein the current signal in at least one electromagnet system has a nonzero magnitude at each instant during said field rotation.

18. A plasma processing apparatus for processing a workpiece, the plasma processing apparatus comprising:

a plasma chamber including an interior region for supporting a plasma;

a plasma generating source;

a vacuum system in fluidic communication with the interior region of the plasma chamber;

a gas supply system in fluidic communication with the interior region of the plasma chamber;

a plurality of coil magnets mounted circumferentially around the plasma chamber, each coil magnet having an axis extending radially from an axis of the plasma chamber;

a plurality of arbitrary waveform generators, each being electrically communicated to an associated one of the plurality of coil magnets;

a control system electrically coupled to the gas supply system, the vacuum system, the cooling system, and the plurality of arbitrary waveform generators, the control system being configured to operate the arbitrary waveform generators so that the coil magnets impose a magnetic field topology on the plasma during the plasma processing operation.

19. A plasma processing apparatus as defined inclaim 18, said plasma generating source comprising one or more electrode assembly mounted within the chamber and one or more RF power sources each electrically coupled to an associated electrode assembly.

20. A plasma processing apparatus as defined inclaim 19, wherein each coil magnet is an air coil.

21. A plasma processing apparatus as defined inclaim 19, wherein each coil magnet has a core of magnetically permeable material.

22. A plasma processing apparatus as defined inclaim 21, further comprising an outer flux conducting structure mounted in surrounding relation to the array of coil magnets, each coil magnet and each core being in magnetic flux communication with the flux conducting structure.

23. A plasma processing apparatus as defined inclaim 22, wherein the flux conducting structure is an annular wall structure.

24. A plasma processing apparatus as defined inclaim 23, wherein the annular wall structure is constructed of a magnetically permeable material.

25. A plasma processing apparatus as defined inclaim 24, wherein each core is mounted on the annular wall structure.

26. A plasma processing apparatus as defined inclaim 18, wherein each arbitrary waveform generator of said plurality thereof is electrically coupled to an associated one of plurality of coil magnets through an associated one of a plurality of amplifiers.

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